Maleic anhydride is a commercially-important compound used in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. n-butane is a relatively inexpensive feed used in the industrial production of maleic anhydride. Improved processes and catalysts for the oxidation of n-butane to maleic anhydride are of interest to the chemical industry.
Mount et al., published application U.S. Ser. No. B330,354 and U.S. Pat. No. 4,111,963, disclose a phosphorus/vanadium/oxygen catalyst and a method of preparing dicarboxylic acid anhydrides by contacting the catalyst with saturated hydrocarbons. More specifically, the Patentees disclose the preparation of maleic anhydride using a phosphorus/vanadium/oxygen catalyst. Oxygen-containing vanadium and phosphorus compounds are contacted to form a catalyst precursor. The importance of reducing the vanadium to valence +4 is described. The precursor is shaped and calcined to form a catalyst. The preferred catalyst has a P/V atom ratio of about 1:2 to 2:1 and a porosity of at least about 35%. The catalyst is contacted with saturated hydrocarbons at a temperature from about 350.degree. to 600.degree. C. to form maleic anhydride.
U.S. Pat. No. 3,864,280 issued to Schneider, discloses the preparation of a phosphorus/vanadium mixed oxide catalyst composition for the oxidation of hydrocarbons. The catalyst is prepared in isobutyl alcohol and has a P/V atom ratio of about 0.9-1.8:1. The vanadium is reduced to an average valence state of 3.9 to 4.6. The catalyst composition is said to have substantial intrinsic surface ara and a microcrystalline structure which result in advantageous activity in the catalyzed vapor phase oxidation of n-butane to maleic anhydride.
U.S. Pat. Nos. 4,062,873 and 4,064,070, issued to Harrison, disclose the preparation of an oxidation catalyst comprising oxides of vanadium, phosphorus, and silicon; and a process of preparing maleic anhydride by contacting the catalyst with hydrocarbon feed at a temperature of about 300.degree. C. to 600.degree. C. A catalyst precursor is prepared by co-precipitating a reduced vanadium oxide, silica or a silica precursor, and phosphoric acid. The precursor is calcined to form the silica-containing catalyst. The preferred vanadium oxidation state is between 3.5 to 4.6. Preferably, the silica is introduced in the form of an organic alkyl orthosilicate. The preferred P/V atom ratio is 0.9-3:1 and the preferred P/Si atom ratio is 20-1:1.
U.S. Pat. Nos. 4,132,670 and 4,187,235, issued to Katsumoto et al., disclose a method of preparing a vanadium/phosphorus oxide catalyst having an intrinsic surface area in excess of 10 square meters per gram. The catalyst is used in the oxidation of n-butane to maleic anhydride. Anhydrous alcohols of 1-10 carbon atoms and 1-3 hydroxyl groups are used to reduce the vanadium to a valence of 4.0 to 4.6. The catalyst has a V/P atom ratio of about 1:1.
U.S. Pat. Nos. 4,371,702 and 4,442,226, issued to Bither, disclose a vanadium/phosphorus oxide catalyst for the oxidation of n-butane to maleic anhydride and containing a promoter comprising silicon and at least one of indium, antimony and tantalum. The catalyst is prepared by initially contacting vanadium substantially of valence +4 with the promoter or promoter precursor in an aqueous or organic liquid medium. In an aqueous system, silica can be added to the vanadium as a colloidal silica sol. In an organic system, silica can be introduced as an alkyl orthosilicate. The patents subsequently disclose that a catalyst precursor is then formed by the addition of an appropriate phosphorus compound to give a composition which upon isolation and drying is blended with 1-3% of a die lubricant and pellet binder, such as graphite or Sterotex.RTM., a hydrogenated cottonseed oil commercially available from Capital City Products Company. The resulting combination is heated to generate the catalyst. The catalyst has a Si/V atom ratio of about 0.02-3.0:1.0, a (In+Sb+Ta)/V atom ratio of about 0.005-0.2:1.0 and a P/V atom ratio of about 0.9-1.3:1.0. The catalyst is contacted with n-butane at 300.degree.-550.degree. C. to form maleic anhydride.
U.S. Pat. No. 4,092,269, issued to Mount et al., discloses a method for preparing phosphorus/vanadium/oxygen catalysts useful in the manufacture of maleic anhydride. A pore modification agent is added to a catalyst precursor to provide a catalyst wherein the pore volume from pores having diameters between about 0.8 .mu.m and about 10 .mu.m is greater than 0.02 cc/g. In the preparation of a preferred catalyst, selected organic pore-modifying agents are incorporated into a catalyst precursor in an amount from about 2-15% by weight of precursor. The modifying agents are removed through combustion.
In any commercial process for the production of maleic anhydride, the product of n-butane conversion and selectivity to maleic anhydride should be maximized. In a process in which the off-gases following recovery of maleic anhydride are combusted, or in a recycle-type of operation, a high single pass yield of maleic anhydride is important.